LeetCode 160. Intersection of Two Linked Lists

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本文介绍了一种在两个单链表中找到它们开始相交节点的方法。通过首先测量每个链表的长度，然后使较长的链表向前移动若干步以平衡长度差距，最后同步遍历两个链表直到找到相同节点或到达末尾。

原题网址：https://leetcode.com/problems/intersection-of-two-linked-lists/

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

If the two linked lists have no intersection at all, return null.
The linked lists must retain their original structure after the function returns.
You may assume there are no cycles anywhere in the entire linked structure.
Your code should preferably run in O(n) time and use only O(1) memory.

方法：根据长度进行检查。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    private int length(ListNode node) {
        int len = 0;
        while (node != null) {
            node = node.next;
            len ++;
        }
        return len;
    }
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        int lenA = length(headA);
        int lenB = length(headB);
        if (lenA >= lenB) {
            for(int i=0; i<lenA-lenB; i++) headA = headA.next;
        } else {
            for(int i=0; i<lenB-lenA; i++) headB = headB.next;
        }
        while (headA != headB && headA != null) {
            headA = headA.next;
            headB = headB.next;
        }
        return headA;
    }
}